The present invention generally relates to radio communication systems, and more particularly, to a radio communication system, a radio transmitter, a radio receiver, and a radio communicating method in which two dimensional interleaving is performed on information symbols in frequency and time axis directions.
In this technical field, OFDM (Orthogonal Frequency Division Multiple access) systems and CDMA (Code Division Multiple Access) systems are promising. The CDMA system distinguishes channels by orthogonal codes to improve interference endurance. The OFDM system selects plural sub-carriers which are orthogonal to each other to improve fading endurance and interference between symbols while effectively utilizing a bandwidth. An OFCDM (Orthogonal Frequency and Code Division Multiplexing) system or a multi-carrier CDMA (MC-CDMA) system that combines the above technologies is known.
In the OFCDM system, information symbols are distinguished by orthogonal spread codes and transmitted on plural sub-carriers. In a radio receiver, necessary information symbols are extracted based on the orthogonality of the spread codes, and recovered by each sub-carrier for demodulation. Accordingly, in order to demodulate in good shape, it is required to maintain the orthogonality between spread codes. However, in a mobile communications system environment, the orthogonality between information symbols may be disturbed because received signal amplitudes or phases representing information symbols are deformed due to the change of propagation path or communication environment. Because of the disturbance of the orthogonality, interference signal components become larger so as to degrade received signal quality, which is a problem.
An “interleaving” technique is known for suppressing the above problem. In this technique, information symbols are rearranged or configured in a pattern before transmitting, and received information symbols are further rearranged or reconfigured in an inverse pattern of the above pattern in a receiver, in order to disperse signal degradation risks and recover the transmitted information symbols accurately. By rearranging the information symbols in frequency axis direction or time axis direction, it is expected to suppress the continuous reception of unreliable signal sequences. This interleaving technology is described in Japanese Laid-Open Patent Application 2002-190788. A two dimensional interleaving technique having a larger interleaving effect is described in Japanese Laid-Open Patent Application 2003-8535.
A radio communication environment is constantly changed over time, and the prior interleaving techniques cannot deal with such constant change in the radio communication environment. In some communication environments, the interleaving method or OFCDM system does not provide the expected result or advantage. As a mobile terminal moves with higher speed and used radio frequencies become higher, such radio communication environmental changes will be more drastic, resulting in significantly degraded signal quality.